We begin tonight with the high drama at the supreme court. Nine justices taking on a central issue of modern american life -- gay marriage. Today the court wrestled with profound questions, what is the family? What is the purpose of marriage? While outside a giant crowd gathered to stand up and argue on all sides. Abc's terry moran was right there inside the courtroom for the historic debate. Reporter: You could sense how conflicted the supreme court justices were about this case -- how they doubted themselves and their power to wade into the marriage debate. Justice anthony kennedy, the likely swing vote, said the court was heading into "uncharted waters," and justice samuel alitod him, noting gay marriage in america is less than ten years old. You want us to step in and render a decision based on an assessment of the effects of this institution which is newer than cell phones or the internet? Reporter: And justice antonin scalia was sharply skeptical that the court could create a constitutional right to gay marriage, after 200 years of laws banning it. When did it become unconstitutional to exclude homosexual couples from marriage? When did the law become this? Reporter: And what about the argument, made by the lawyer defending california's proposition 8, which bans gay marriage, that the true purpose of marriage is procreation? Well, suppose a state said that, because we think that the focus of marriage really should be on procreation, we are not going to give marriage licenses anymore to any couple where both people are over the age of 55. Reporter: And the court considered the children, too -- 40,000 being raised by gay couples in california alone -- justice kennedy spoke emotionally about their interests. They want their parents to have full recognition and full status. The voice of those children is important in this case, don't you think? Reporter: For the two gay couples at the heart of the case -- kristin perry and sandra stier, jeff zarillo and paul katami -- this was their families' moment. To be able to call him my husband is something that has worldwide recognition. When I am able to introduce him that way, people understand the level of commitment. Reporter: And this day was about the kids, too. Outside afterwards, spencer perry, one of kris perry and sandra stier's four boys, stepped to the microphones. I just want to say how incredibly proud we are of our parents. We love them. We love our family, and we look forward to the day when we will be treated equally, just like our neighbor's family. Those emotions, the whole debate on this issue, all that was in the courtroom. And the one dominant sense they got from the justices, was that they want that great debate to continue. They don't want to cut it off with some ruling awarding total victory to one side or the other. But they will continue a debate tomorrow on the same issue from a different vantage point? A different angle. Today's case was about a state law banning gay marriage. Tomorrow's case about that federal law, that says no matter what a state does, even in a state recognizes gay marriage, the federal government won't. So gay couples are just people according to the federal government, who have no right to marry.

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